Increasing boiler output with oxygen

ABSTRACT

A fuel-fired steam generation apparatus such as a boiler can produce steam at a rate higher than the maximum rate at which it can produce steam using air as the only source of oxygen for combustion, by combusting the fuel with oxidant having a higher oxygen content than air but feeding the oxidant at a volumetric flow rate lower than the rate at which maximum steam production is obtained with air as the only source of oxygen for combustion.

FIELD OF THE INVENTION

This invention relates to boilers and like devices in which fuel iscombusted to generate heat which is transferred to water passing throughthe device.

BACKGROUND OF THE INVENTION

Fuel-fired boilers that combust fuel and transfer the heat generated bythe combustion to water that is fed through the apparatus so as to heatthe water, evaporate it into steam, and heat the steam, typically arelimited in the maximum amount of steam that they can produce. Themaximum amount of steam that can be produced is typically limited by thevolume of the combustion chamber, and by the properties of the apparatusitself including the heat exchangers that are employed to heat the waterand the steam. For example, a fuel-fired boiler typically faces at leastthe following limitations on the rate at which steam can be produced:limitations in the capacity of the fans which drive combustion air intothe apparatus, and which assist passing flue gas out of the device; thetemperature of the flue gas, particularly as it first exits thecombustion chamber, since excessive temperatures risk degrading thematerial of construction of heat exchangers that come in contact withthe hot flue gas, and risk raising the temperature of water or steampassing through the heat exchangers so high that the heat exchanger mayrupture or otherwise fail from within; and the capacity of devices, suchas those known as attemperators, which are used to reduce thetemperature of the heated steam that is generated by the apparatus.

Thus, attempts to increase the mass flow rate of steam produced by theapparatus, simply by increasing the rate at which fuel is fed to theapparatus, or by increasing the amount of water fed to the apparatus,are eventually limited by one or more of the foregoing constraints, whenair is employed as the source of oxygen for combustion of the fuel.

This situation is also affected by the fact that in many cases, thesteam that is produced by the apparatus must be provided at atemperature which is within a given temperature range generally 5° F.above or below that given temperature. This limitation is imposedparticularly in situations in which the steam produced by the apparatusis to be directed into turbines for generating electrical power. Suchturbines are generally designed on the basis of several significantinputs, one of which is the temperature of the incoming steam into theturbine.

Other attempts have been made to provide increased steam generatingcapability, such as flue gas recirculation, and such as rebuilding allor part of the steam generating apparatus, but these attempts imposeoperational and economic burdens which operators want to avoid.

Accordingly, there remains a need for technology to increase thecapability of a given, already existing steam generating apparatus toproduce steam at higher mass flow rates, at the given desired steamtemperature. The present invention provides this capability.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method of modifying a steamgeneration apparatus, comprising

(A) providing a steam generation apparatus comprising

(1) a combustion chamber into which gaseous oxidant and fuel are fed,and in which said fuel and said gaseous oxidant are combusted therebyproducing heat and hot flue gas,

(2) a preheater into which and through which said hot flue gas passesfor transferring heat to said gaseous oxidant before said gaseousoxidant is fed to said combustion chamber, and

(3) a pathway into which water is fed, from which heated steam isobtained at an outlet, and within which the water is evaporated andconverted to said heated steam by heat transferred thereto in one ormore heat exchangers in said pathway and within said steam generationapparatus to which heat of said combustion is radiated and in one ormore convective heat exchangers in said pathway and within said steamgeneration apparatus to which heat in said hot flue gas is transferredby convective heat exchange,

the pathway further comprising one or more devices that can pass coolantinto steam in the pathway to reduce the temperature of said steam,

wherein the highest mass flow rate at which said apparatus can providesteam at said outlet at a temperature within a given temperature range,when said gaseous oxidant is air and said air is the only source ofoxygen for said combustion, is limited by the maximum flue gastemperature to which said heat exchangers can be exposed, by maximumrates at which said fuel can be fed into said combustion chamber and atwhich said flue gas can be withdrawn therefrom, and by the maximum rateat which said coolant can be passed into said steam, and

(B) increasing the rate at which steam is provided at said outlet at atemperature within said given temperature range to a mass flow ratehigher than said highest mass flow rate, by

(1) increasing the mass flow rate into said pathway of water by anamount equal to the desired increased flow rate of steam and increasingthe mass flow rate of fuel into said combustion chamber by an amountcorresponding to the increased amount of heat needed to produce saidsteam at said increased rate, while

(2) increasing the oxygen content of said gaseous oxidant with whichsaid fuel is combusted but decreasing the volumetric flow rate at whichsaid gaseous oxidant having increased oxygen content with which saidfuel is combusted is fed into said combustion chamber to valueseffective to provide said increased mass flow rate of steam withoutexceeding any of said maxima, and

(3) combusting said fuel in said combustion chamber with said gaseousoxidant having said increased oxygen content fed at said decreasedvolumetric flow rate,

without decreasing or increasing the heat transfer area of any of saidheat exchangers,

wherein flue gas produced in said combustion chamber that exits fromsaid apparatus is not fed back into said apparatus, and the temperatureof said flue gas entering said preheater is lower than when said fluegas is produced by combustion in said apparatus when air is the onlysource of oxygen for combustion.

Another aspect of the present invention is a method of operating a steamgeneration apparatus, comprising

(A) providing a steam generation apparatus comprising

(1) a combustion chamber into which gaseous oxidant and fuel are fed,and in which said fuel and said gaseous oxidant are combusted therebyproducing heat and hot flue gas,

(2) a preheater into which and through which said hot flue gas passesfor transferring heat to said gaseous oxidant before said gaseousoxidant is fed to said combustion chamber, and

(3) a pathway into which water is fed, from which heated steam isobtained at an outlet, and within which the water is evaporated andconverted to said heated steam by heat transferred thereto in one ormore heat exchangers in said pathway and within said steam generationapparatus to which heat of said combustion is radiated and in one ormore convective heat exchangers in said pathway and within said steamgeneration apparatus to which heat in said hot flue gas is transferredby convective heat exchange,

the pathway further comprising one or more devices that can pass coolantinto steam in the pathway to reduce the temperature of said steam,

wherein the highest mass flow rate at which said apparatus can providesteam at said outlet at a temperature within a given temperature range,when said gaseous oxidant is air and said air is the only source ofoxygen for said combustion, is limited by the maximum flue gastemperature to which said heat exchangers can be exposed, by maximumrates at which said fuel and gaseous oxidant can be fed into saidcombustion chamber and at which said flue gas can be withdrawntherefrom, and by the maximum rate at which said coolant can be passedinto said steam, and

(B) providing steam at said outlet at a temperature within said giventemperature range at a desired mass flow rate which is higher than saidhighest mass flow rate, by

(1) feeding water into said pathway at a mass flow rate equal to thedesired mass flow rate of steam and feeding fuel into said combustionchamber at a mass flow rate corresponding to the amount of heat neededto produce said steam at said desired mass flow rate, while

(2) feeding into said combustion chamber gaseous oxidant with which saidfuel is combusted having a higher oxygen content than air, at avolumetric flow rate lower than the volumetric flow rate of air at whichsaid highest mass flow rate of steam is provided when air is the onlysource of oxygen for said combustion, at values of said oxygen contentand said volumetric flow rate which are effective to provide saiddesired mass flow rate of steam without exceeding any of said maxima,and

(3) combusting said fuel in said combustion chamber with said air andsaid gaseous oxidant having said increased oxygen content fed at saidlower volumetric flow rate,

wherein the heat transfer area of said heat exchangers is the same aswhen air is the only source of oxygen for said combustion, and whereinflue gas produced in said combustion chamber that exits from saidapparatus is not fed back into said apparatus, and the temperature ofsaid flue gas entering said preheater is lower than when said flue gasis produced by combustion in said apparatus when air is the only sourceof oxygen for combustion.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic side cross-section of a steam generating apparatuswith which the present invention is useful.

FIG. 2 is a schematic side cross-section of another steam generatingapparatus with which the present invention is useful.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a simplified schematic view is shown of a steamgeneration apparatus 1, a boiler, with which the present invention canbe practiced.

Fuel 3 and gaseous oxidant 5 are fed to burner 7. The fuel and theoxidant combust as flame 9 in combustion chamber 10. The combustionforms hot flue gas 12 which passes out of combustion chamber 10 andflows in convective heat exchange contact with heat exchangers asdescribed below. Preferably, before the gaseous oxidant is fed intoburner 7 it is heated in preheater 15 by heat transfer there from hotflue gas 12 entering preheater 15.

The fuel can be any combustible solid, liquid or gas, or mixturesthereof. The preferred fuel is coal Other preferred fuels with whichthis invention can be practiced include other combustible solid matter,liquids such as fuel oil, and gases such as methane and natural gas.

Solid fuel such as coal is typically fed as a stream of pulverizedsolids carried in a stream of transport air through the burner into thecombustion chamber. Liquid fuels are typically atomized, and theatomizing can be carried out by a stream of air which thus enters intothe combustion chamber together with the liquid fuel.

Burners employed for combustion of any of these fuels typically includeinlets in close proximity to the point at which the fuel enters into thecombustion chamber, through which combustion air (known as “primaryair”) enters into the combustion chamber and to provide oxygen forcombustion with the fuel. As is known, some burner designs containadditional passages in which additional air, known as “secondary air”,can also enter into the combustion chamber and participate in thecombustion of the fuel.

Typically, combustion air is provided to the burner air inlets, and toevery burner in apparatus having more than one burner, from a commonsource by appropriate feed lines. Another typical arrangement is thatthe combustion air is fed from its source to a windbox from which theair flows directly to each burner.

The boilers and other steam generation devices to which this inventionapplies can be configured so that the fuel and all gaseous oxidant (e.g.combustion air) needed for complete combustion of the fuel entersthrough burners such as burner 7 installed in the radiative section ofthe combustion chamber 10. Alternatively, the devices can be configuredso that only a portion of the gaseous oxidant (e.g. combustion air)enters through burners such as burner 7, and the balance thereof(typically 10% up to 25% of the total oxygen for combustion fed to theapparatus) enters through one or more overfire ports 17 locatedvertically above said burners, between the radiative zone burners andthe convective section heat exchangers.

Feed water 21 that is to be converted to steam follows a pathway(through appropriate piping and heat exchangers) from a suitable source,through optional but preferred heat exchanger 22 (known as an“economizer”) in which it is heated by heat transferred from hot fluegas 12, and through one or more heat exchangers 24 where it is heated byheat radiated from the combustion in combustion chamber 10, and throughone or more convective heat exchangers (such as at 26 and 28, also knownas “superheaters”) which transfer heat by convective heat exchange fromthe hot flue gas. In a typical steam generation apparatus, there is oneor more device 40 (commonly known as an “attemperator”) which can feedcoolant such as water into the stream of hot steam in order to reducethe temperature of the steam as desired by the operator. As describedabove, reducing the temperature of the steam can become necessary inorder to provide the steam at its outlet 30 at a temperature in thedesired temperature range appropriate for further use of the steam as ina turbine. Typically, depending on the apparatus, the desiredtemperature may be 800° F. to 1200° F., often about 1000° F., so thedesired range being plus or minus 5° F. from the given desiredtemperature. It should be understood that outlet 30 is not confined to anozzle or the like, but can be a point in a conduit that conveys thesteam to another piece of equipment such as a turbine.

Steam generating apparatus of this type can also be used to heat steamreceived from other sources. In one such embodiment, the apparatus heatssteam (termed “reheat steam”) that has been exhausted from a powerturbine, especially where the power turbine is a power turbine to whichsteam from outlet point 30 was fed. FIG. 2 illustrates such anembodiment, wherein steam 50 is passed through heat exchangers 52 and 54in which the steam is heated by heat transfer from flue gas 12, and isprovided point 60 at a desired temperature. Device 56, which istypically a controllable supply of attemperating water, can injectcoolant into the steam in order to reduce the temperature of that steamso that its temperature at outlet point 60 is as desired.

The present invention is also useful in steam generating apparatuswherein burners are arrayed in several rows, typically 2 to 10 rows,with each row containing 2 to 10 or more burners side by side in eachrow. In these embodiments, at least one burner is at a higher elevationthan at least one other of said burners, with the flue gas exiting thecombustion chamber at a higher elevation than even the uppermost of allthe burners. Preferably, each burner at a given elevation is one of aplurality of burners in a given row of burners at the same elevation. Inthese embodiments, the same amount of fuel and oxidant can be fed toevery burner. Preferably, though, to a “lower group” of burners whichcontains half or more than half of the total number of burners (but lessthan all of the burners), and in which all the burners in this lowergroup are at lower elevations than all of the burners that are not inthis lower group; more than half of the total amount of fuel that is fedto all burners of the apparatus is fed to burners that are in this“lower group”. (The “lower group”, or the group of burners not in this“lower group”, or both groups, may contain one or more than one burner.)In this embodiment, illustrated in Case 5 in the Example, the rate ofproduction of steam is increased, and operation is so efficient that nocooling fluid (such as attemperating water) at all is fed into the steamthat is produced in this embodiment.

It has been determined by simulations of the operation of a coal-firedsteam generating apparatus of the type depicted in FIG. 1, that steamcan be produced at a mass flow rate higher than the maximum mass flowrate attainable using air as the only source of oxygen for combustion ofthe fuel, but at the same given target temperature as attained using airas the only source of oxygen for combustion, as follows:

Additional water is fed to the apparatus, so that the total amount ofwater fed equals the total amount of steam that is to be provided at theoutlet. Additional fuel is provided into the apparatus, in an amountcorresponding to the additional amount of steam that is to be providedat the given temperature, so that the total amount of fuel fed to theapparatus corresponds to the amount of steam to be provided under thesenew operating conditions.

The amounts of fuel and oxidant to be provided can be determined for anygiven operation from straightforward stoichiometric and thermodynamiccalculations.

In addition, two adjustments are made to the oxidant with which the fuelis to be combusted.

One adjustment is that the oxygen content of the oxidant is increased.If the apparatus had been operating with air as the only source ofoxygen for combustion, then the oxygen content of the oxidant isincreased above that of air. However, even if the overall oxygen contentof oxidant provided to the apparatus to be combusted with the fuel wasat a value already above that of air, then the oxygen content isincreased above that value.

The oxygen content can be increased in any of several ways. One way isto premix oxygen, or highly-oxygen-enriched air, with the incoming air,before or after the air passes through the preheater, so that oxidantcontaining the desired higher oxygen content reaches all burners.Another technique is to feed oxygen, or highly-oxygen-enriched air, intothe windbox when a windbox is employed with the apparatus as describedabove. A third technique is to inject, with a lance or other suitableapparatus, high purity oxygen directly into the burner, or into eachburner if more than one burner is employed in the apparatus.

While in principle any higher oxygen content is effective in conjunctionwith the other adjustments described herein, the final oxygen content ofthe oxidant can typically be up to 30 vol. %, but the advantages of thepresent invention can be realized if the oxygen content of the oxidantis up to 25 vol. %, or less.

Another adjustment that is made in the supply of oxidant to thecombustion is that the overall volumetric flow rate of the oxidant isreduced, typically by up to 20 percent from the volumetric flow ratethat had been employed in obtaining the maximum mass flow rate of steamobtainable using air as the only source of oxygen for combustion.

However, the volumetric flow rate of oxidant may be reduced from alesser volumetric flow rate as well, particularly in the course ofbalancing that flow rate with the increased oxygen content of theoxidant being fed at that reduced volumetric flow rate.

A preferred technique is to replace a given volume of air from theoxidant with that same volume of oxygen of at least 90 vol. %, andpreferably at least 99.9 vol. %, purity. This results in reducing thetotal volume of oxidant being fed, while increasing the oxygen contentof that oxidant.

The steam generating apparatus is then operated with combustion carriedout using oxidant having an oxygen content higher than that of air, butwhich is fed at a volumetric flow rate lower than the rate at which airwas being fed to the apparatus when the apparatus was generating itsmaximum possible mass flow rate of steam at the aforesaid giventemperature with air as the only source of oxygen for combustion.

Under these conditions, surprisingly, steam is provided at a higher massflow rate than the maximum mass flow rate of steam that could beobtained using combustion with air as the only source of oxygen, but theaforesaid operational maximums are not exceeded. That is, the maximumflue gas temperature that can be tolerated by the apparatus, includingparticularly by the heat exchangers with which the flue gas comes intocontact, is not exceeded, even though more heat is generated and steamis produced at a higher mass flow rate.

This discovery is surprising in that one would normally expect thatfeeding additional amounts of fuel, and feeding oxidant containing moreoxygen than air, into the same apparatus without changing the heattransfer surface areas of any of the heat exchangers and withoutotherwise changing the physical dimensions of the apparatus itself,would lead to generation of such higher amounts of heat and such highertemperatures that the apparatus would not be able to tolerate suchconditions. However, the results described in the following Exampleindicate that the aforementioned maxima are not exceeded, and indeedhigher rates of steam production are attained. It is even moresurprising, and beneficial, that the amount of attemperating water orother coolant to add to the steam are reduced and even completelyeliminated.

Without intending to be bound by any particular explanation for theseobservations, the data reported in the Example are consistent with theproposition that enough heat produced by combustion of the fuel and theoxidant having a higher oxygen content than air is conveyed by radiativeheat transfer to the one or more heat exchangers to which heat ofcombustion is radiated, and is thus transferred in those heat exchangersto the water, that the temperature and heat content of the flue gasexiting the combustion chamber do not exceed the maximum that is imposedby the equipment including the other heat exchangers with which the hotflue gas comes into contact; and the subsequent heat exchange from theflue gas to those other heat exchangers raises the temperature of thesteam even higher, as the water had already absorbed that additionalamount of radiative heat from the combustion.

In the embodiments in which some combustion air is provided into thecombustion chamber as the aforementioned overfire air, the oxygencontent of that air provided as overfire air should be less than theoxygen content of the oxidant supplied to the burner.

Among other advantages, the flue gas temperature at the point at whichthe flue gas enters the aforementioned preheater is lower than is thecase when the apparatus is operated using air as the only source ofoxygen for combustion. This is advantageous in that it reduces thefurther need to cool the flue gas. This also confirms that more of theheat contained in the flue gas has been transferred to the production ofsteam.

EXAMPLE

The invention is described in detail for ten cases of boiler operationsimulated by a computer model. The boiler is assumed to be of the typeillustrated in FIG. 2, with the feature that reheat steam exhausted froma power turbine is passed through the apparatus to be reheated by heatexchange with flue gas 12. The boiler computer model is based on zonemethods of analysis. The model divides the boiler furnace into hundredsor thousands of volume and surface zones. It then calculates local andoverall heat transfer, gas and wall temperature profiles, and volatileand char burn out, depending on coal properties and boiler operatingconditions. Boiler steam production is calculated using computerizedsteam tables after the net heat transfer to various radiative andconvective heat exchangers are known. This modeling approach for fuelcombustion and flame radiation had been employed to model air- and oxy-fuel fired glass furnaces with success and are described in Wu, K. T.and Kobayashi, H., “Three-Dimensional Modeling of AlkaliVolatilization/Crown Corrosion in Oxy-Fired Glass Furnaces”, 98^(th)Annual Meeting of the American Ceramic Society, Indianapolis, Ind.,1996.

The simulations assumed a 278 MW thermal input wall-fired boiler firedwith bituminous coal using four rows of burners, spaced one row abovethe other, with four burners in each row. Table 1 summarizes propertiesof the coal used in the modeling simulation.

TABLE 1 Proximate Analysis (%, wet) Moisture 7.9 Volatile Matter 37.1Fixed Carbon 43.6 Ash 11.4 HHV (Btu/lb, wet) 12885 Ultimate Analysis (%,dry) C 72.3 H 4.6 N 1.3 O 7.7 S 1.7 Ash 12.4

Table 2 summarizes five simulated cases in which the boiler is operatedwithout overfire air. Case 1 is based on data taken from operation of anactual power generation boiler. For simulated Cases 2 through 5, maximumcoal flow and therefore maximum steam generating capacity is reachedwhen the temperature of the flue gas entering the screen tubes (thepoint at which the flue gas first contacts a convective heat exchanger)reaches 2375±3 F. The five cases in Table 2 are:

Case 1. Baseline operation with air only. All burners fired at the samefiring rate.Case 2. Maximum steam generation capacity operation with air only. Allburners are fired equally.Case 3. Steam generation rate increased by oxygen enrichment ofcombustion air to 22.09% O₂ in air. All burners are fired equally.Case 4. Steam generation rate increased by oxygen enrichment ofcombustion air to 22.80% O₂ in air. All burners are fired equally.

Case 5. Same as Case 4, but heat input biased so that 35% of the fuel isfed and combusted in each of the bottom two rows of burners, and 15% ofthe fuel is fed and combusted in each of the top two rows of burners.

TABLE 2 Boiler Operations without overfired oxidant Case 1 Case 2 Case 3Case 4 Case 5 Furnace Operation: Coal flow (lb/hr) 73551 84584 8458484768 88262 Firing rate (MMBtu/hr, HHV) 947 1089 1089 1092 1137 Oxidantflow (SCFH) 11016934 12667944 11687674 11274010 11738744 O2% in oxidant20.67 20.67 22.09 22.80 22.80 Flue gas flow (SCFH) 11518689 1324497512264705 11852286 12340857 Burner load equal equal equal equaldistributed Air preheat (F.) 600 631 616 610 617 Flue Gas Temperatures(F.): Entering screen tubes 2271 2374 2378 2373 2375 Entering finishingsuperheater 2093 2198 2191 2183 2190 Entering economizer 829 882 855 842853 Entering air preheater 696 740 716 706 715 Heat Absorptions(MMBtu/hr): Waterwalls 24 470 509 544 561 585 Finishing superheater 2851 60 58 59 59 Reheaters 52 and 54 106 128 122 118 122 Steam Production:Superheated steam (lb/hr) 693396 784238 798178 809028 843005 Reheatedsteam (lb/hr) 623938 712958 717869 722858 752400 Superheat attemperation(lb/hr) 10755 36622 9171 0 0 Reheat attemperation (lb/hr) 6803 150087437 2827 2178 Superheat attemperation (% 1.6 4.7 1.1 0.0 0.0superheated steam) Reheat attemperation (% 1.1 2.1 1.0 0.4 0.3 reheatsteam) Carbon in Ash (%): 6.81 7.82 6.95 6.64 6.54 Boiler Efficiency:Gross (% of HHV coal heat 89.17 88.43 89.22 89.41 89.25 input)

Table 3 summarizes five simulated cases in which combustion is staged byoperating the boiler with overfire air. In the simulations, maximum coalflow and therefore maximum boiler steam generating capacity is reachedwhen the flue gas temperature entering the screen tubes reached 2375±3F. The five cases in the table are:

Case 6. Baseline operation with air. All burners are fired equally.Burner zone SR (stoichiometric ratio, the ratio of oxygen fed to theamount of oxygen needed to completely combust all combustible componentsin the fuel) is 0.9 and only air is used in the overfire zone.Case 7. Maximum steam generation capacity operation with air only. Allburners are fired equally. Burner zone SR is 0.9 and only air is used inthe overfire zone.Case 8. Steam generation rate increase by oxygen enrichment ofcombustion air to 22.09% O₂ in air. All burners are fired equally.Burner zone SR is 0.9 and oxygen enrichment is applied to both theburner combustion air and the overfire air.Case 9. Same as Case 8 except the burners are fired at different loadsand SR levels. Oxygen enrichment is applied to both the burnercombustion air and the overfire air.Case 10. Same as Case 8 except the burners are fired at different firingrates and SR levels. Oxygen enrichment is applied only to the burnercombustion air, not the overfire air.

TABLE 3 Boiler operations with overfired oxidant Case 6 Case 7 Case 8Case 9 Case 10 Furnace Operation: Coal flow (lb/hr) 73551 84510 8311382230 85320 Firing rate (MMBtu/hr, HHV) 947 1088 1070 1059 1099 Oxidantflow (SCFH) 11016934 12656909 11484452 11362430 12194070 O2% in oxidant20.67 20.67 22.09 22.09 22.09/20.67 Flue gas flow (SCFH) 1151868913233444 12051452 11923404 12685502 Burner load equal equal Equaldistributed distributed SR1/SRt 0.90/ 0.90/ 0.90/1.22 0.75 to 1.0/ 0.75to 1.0/ 1.24 1.24 1.22 1.23 Air preheat (F.) 602 633 616 613 631 FlueGas Temperatures (F.): Entering screen tubes 2262 2374 2374 2376 2377Entering finishing superheater 2083 2200 2189 2186 2191 Enteringeconomizer 833 886 854 850 870 Entering air preheater 698 743 714 711729 Heat Absorptions (MMBtu/hr): Waterwalls 24 459 495 522 519 523Finishing superheater 28 51 61 59 58 60 Reheaters 52 and 54 107 131 123122 127 Steam Production: Superheated steam (lb/hr) 685476 775051 775130768240 790733 Reheated steam (lb/hr) 619106 708602 701237 694822 716681Superheat attemperation (lb/hr) 16561 45881 17479 15286 27174 Reheatattemperation (lb/hr) 9005 18802 11373 11136 12941 Superheatattemperation (% 2.4 5.9 2.3 2.0 3.4 superheated steam) Reheatattemperation (% 1.5 2.7 1.6 1.6 1.8 reheat steam) Carbon in Ash (%):11.20 11.66 10.30 10.33 11.20 Boiler Efficiency: Gross (% of HHV coalheat 88.42 87.88 88.60 88.74 88.18 inputs)

The most general illustration of this invention as applied to a boilerwhere all combustion air is supplied through the burners (that is, aboiler without overfire air) is made by comparing modeling Cases 3 and 4to Case 2. These comparisons show that the mass flow rate of steam andthe efficiency can be increased by increasing the oxygen content of thegaseous oxidant, such as by enriching the combustion air with oxygen, orby introducing oxygen into the combustion zone of the boiler by somemeans other than general enrichment (such as lancing). They also showthat using oxygen to increase the rate of steam production brings addedbenefits of reduced attemperation coolant flows and reduced flue gastemperature at the inlet to the air preheater relative to maximumcapacity operation with air as the only source of oxygen for combustion.The modeling of this invention indicates capacity and efficiency gainscan be achieved with oxygen enrichment without problems arising relatedto heat transfer imbalance and therefore without having to resort toflue gas recirculation to the combustion chamber, and the complicationsand drawbacks associated with flue gas recirculation.

Referring to Table 2, in Case 3, oxygen is used to increase combustionair oxygen concentration from 20.67% oxygen to 22.09% oxygen. Thisenrichment results in the ability to produce more superheat (and, ifdesired, reheat) steam without exceeding the screen tube temperaturelimit than is possible in the cases without oxygen enrichment as in Case2. It also results in an increase in boiler fuel efficiency (89.22%efficiency in Case 3 versus 88.43% efficiency in Case 2).

Other benefits of using oxygen are lower attemperation coolant flowrates and reduced flue gas temperature at its inlet to the airpreheater. In Case 2 attemperation spray flow and air preheater gasinlet temperature are increased relative to Base Case 1, due to theincrease in flue gas flow rate that results from increasing the firingrate but using only air as the source of oxygen for combustion. Theincreased flue gas flow carries more heat to the convective heatexchangers of the boiler. This raises the gas temperature at the airpreheater inlet, and increases the steam temperature which in turnnecessitates increased attemperation coolant flow rates for cooling thesteam. As noted above, attemperation coolant flow rates and the airpreheater flue gas inlet temperature can be capacity limiting factors ina boiler. For Case 3, which employs oxygen enrichment, attemperationcoolant flow rates and the air preheater flue gas inlet temperature arelower than in either Case 1 or Case 2, possibly because of the reducedflue gas volume and heat content that results from replacement of aportion of the combustion air with oxygen. As shown in Table 4, thereduced requirement for steam temperature reduction (reduced“desuperheating load”) results in reduced attemperation coolant flows.

TABLE 4 Steam Production Rate Increase Case 2 Case 3 Superheated SteamTemperature (F.) of steam to 955 898 which attemperating water is addedat 40 Temperature (F.) of steam 874 879 after addition of attemperatingwater at 40 Amount of de-superheating, F. 81 19 Amount of attemperating36622 9171 water flow, lb/h Rate of production of 784238 798178superheated steam at outlet 30, lb/h Reheat Steam Temperature (F.) ofsteam to 826 818 which attemperating water is added at 56 Temperature(F.) of steam 786 798 after addition of attemperating water at 56 Amountof de-superheating, F. 40 20 Amount of attemperating 15008 7437 waterflow, lb/h Rate of production of 712958 717869 reheated steam at outlet60, lb/h

Case 4 shows the result of further oxygen enrichment of the oxidant, toan oxygen content of 22.8 vol. %. The steam generation rate and boilerefficiency are increased further versus Case 3 at this higher level ofenrichment, but still without exceeding the temperature limit for theflue gas at the screen tubes, and the attemperation coolant flow ratesand air preheater flue gas inlet temperature are further reduced.

Another embodiment of using lower flow rates of oxidant having an oxygencontent higher than that of air, in a boiler without overfire air, isillustrated by Case 5. In Case 5, oxygen addition to the oxidant iscombined with distributing a greater portion of total fuel input to thelower rows of burners. This fuel redistribution allows the total firingrate to be increased further and allows the steam generation rate to beincreased without exceeding the temperature limit for the flue gasentering the screen tubes. Replacing a portion of the combustion airwith oxygen allows greater fuel redistribution than is possible with airas the only source of oxygen.

Oxygen addition results in flame stability at a wider range of velocityand mixing conditions, and also results in reduced pressure drop throughthe burner combustion air passages thereby allowing higher firing rateoperation.

In Case 5, the oxygen content of the combustion air is 22.9 vol. %oxygen. Additionally, fuel input is redistributed so the bottom two rowsof burners each supply 35% of total heat input, and the upper two rowsof burners each supply 15% of total heat input. The maximum steamgeneration rate, as determined by reaching the maximum allowable fluegas temperature at the screen tubes, is greater than that in any ofCases 2, 3 and 4, all of which have equal distribution of fuel to allrows of burners. Boiler efficiency in Case 5 is higher than theefficiency in Case 2 which is the maximum capacity case for operationwith air as the only source of oxygen for combustion. The attemperationcoolant flow rate is low relative to Cases 2, 3 and 4, and the flue gastemperature at its inlet to the air preheater is low relative to Cases 2and 3.

Cases 6 through 10 apply to a boiler in which a portion of thecombustion air flow is diverted from the burners to a separate overfireair injection port for injection of air into the boiler above the toprow of burners and below the convective heat exchangers.

In Case 6, the firing rate matches that of Base Case 1. 90% of theamount of air required for complete stoichiometric combustion issupplied through the burners (at which the SR=0.9) and additional air issupplied as overfire air to bring the total boiler stoichiometric ratioto 1.24.

Case 7 calculates the maximum steam generation rate for the boiler whenit is operated with overfire air, determined as the rate when themaximum permissible flue gas temperature entering the screen tubes isreached. The same burner zone and stoichiometric ratios apply as in Case6. Attemperation coolant flow rates and the flue gas temperature at theinlet to the air preheater are increased substantially relative to Case6, and could prevent actual achievement of this steam generation rate inan actual boiler.

In Case 8, the oxygen concentration in the oxidant for combustion, andin the oxidant fed through the overfire air port, is 22.09% oxygen. Thehigher oxygen content in the combustion oxidant and at the overfire airport does not yield an increase in the steam generating rate. Theproduction rate of superheated steam changes insignificantly, but about1% less reheat steam is produced relative to Case 7. There is anincrease in boiler fuel efficiency (88.6% efficiency in Case 8 versus87.88% efficiency in case 7), but the objective of increased steamproduction rate is not achieved. However very close to the same capacityas in Case 7 is achieved with significantly lower attemperation coolantflows and with lower temperature at the flue gas inlet to the airpreheater.

In Case 9, burner and overfire combustion air are again 22.09 vol. %oxygen. Additionally, fuel input is redistributed so that the top threerows of burners each supply 22.2% of total fuel input, and the bottomrow of burners supplies 33.3% of total fuel input. The stoichiometricratio of the lowest row of burners is 1.0, and of the remaining burnersis 0.75. The oxygen in the overfire air flow brings the total boilerstoichiometric ratio to 1.22. With this oxygen addition and fueldistribution, the maximum steam generation rate as determined byreaching the maximum allowable flue gas temperature at the screen tubesis reduced relative to Case 7 (which had no oxygen enrichment).

Case 10 is identical to Case 9 with the exception that the oxygencontent of the oxidant is higher only in the oxidant fed to the burners,not in the air fed through the overfire port. In this embodiment, boththe steam generation rate and boiler efficiency are increased relativeto Case 7, the case of maximum steam production without oxygen addition.The increased steam generation rate in Case 10 is achieved with lessattemperation coolant flow, and with a lower flue gas temperature at theair preheater inlet, than in Case 7.

1. A method of modifying a steam generation apparatus, comprising (A) providing a steam generation apparatus comprising (1) a combustion chamber into which gaseous oxidant and fuel are fed, and in which said fuel and said gaseous oxidant are combusted thereby producing heat and hot flue gas, (2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber, and (3) a pathway into which water is fed, from which heated steam is obtained at an outlet, and within which the water is evaporated and converted to said heated steam by heat transferred thereto in one or more heat exchangers in said pathway and within said steam generation apparatus to which heat of said combustion is radiated and in one or more convective heat exchangers in said pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred by convective heat exchange, the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam, wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and (B) increasing the rate at which steam is provided at said outlet at a temperature within said given temperature range to a mass flow rate higher than said highest mass flow rate, by (1) increasing the mass flow rate into said pathway of water by an amount equal to the desired increased flow rate of steam and increasing the mass flow rate of fuel into said combustion chamber by an amount corresponding to the increased amount of heat needed to produce said steam at said increased rate, while (2) increasing the oxygen content of said gaseous oxidant with which said fuel is combusted but decreasing the volumetric flow rate at which said gaseous oxidant having increased oxygen content with which said fuel is combusted is fed into said combustion chamber to values effective to provide said increased mass flow rate of steam without exceeding any of said maxima, and (3) combusting said fuel in said combustion chamber with said gaseous oxidant having said increased oxygen content fed at said decreased volumetric flow rate, without decreasing or increasing the heat transfer area of any of said heat exchangers, wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.
 2. A method according to claim 1 wherein said gaseous oxidant fed into said combustion chamber in step (A)(1) is air.
 3. A method according to claim 1 wherein said fuel is coal.
 4. A method according to claim 1 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.
 5. A method according to claim 1 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising feeding more than half of the total amount of fuel that is fed to all of said plurality of burners to a group of burners which contains at least half but fewer than all of said burners, in which group all burners are at a lower elevation than all burners not in said group.
 6. A method according to claim 1 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.
 7. A method according to claim 1 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners.
 8. A method of operating a steam generation apparatus, comprising (A) providing a steam generation apparatus comprising (1) a combustion chamber into which gaseous oxidant and fuel are fed, and in which said fuel and said gaseous oxidant are combusted thereby producing heat and hot flue gas, (2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber, and (3) a pathway into which water is fed, from which heated steam is obtained at an outlet, and within which the water is evaporated and converted to said heated steam by heat transferred thereto in one or more heat exchangers in said pathway and within said steam generation apparatus to which heat of said combustion is radiated and in one or more convective heat exchangers in said pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred by convective heat exchange, the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam, wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel and gaseous oxidant can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and (B) providing steam at said outlet at a temperature within said given temperature range at a desired mass flow rate which is higher than said highest mass flow rate, by (1) feeding water into said pathway at a mass flow rate equal to the desired mass flow rate of steam and feeding fuel into said combustion chamber at a mass flow rate corresponding to the amount of heat needed to produce said steam at said desired mass flow rate, while (2) feeding into said combustion chamber gaseous oxidant with which said fuel is combusted having a higher oxygen content than air, at a volumetric flow rate lower than the volumetric flow rate of air at which said highest mass flow rate of steam is provided when air is the only source of oxygen for said combustion, at values of said oxygen content and said volumetric flow rate which are effective to provide said desired mass flow rate of steam without exceeding any of said maxima, and (3) combusting said fuel in said combustion chamber with said air and said gaseous oxidant having said increased oxygen content fed at said lower volumetric flow rate, wherein the heat transfer area of said heat exchangers is the same as when air is the only source of oxygen for said combustion, and wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.
 9. A method according to claim 8 wherein said fuel is coal.
 10. A method according to claim 8 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.
 11. A method according to claim 8 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising feeding more than half of the total amount of fuel that is fed to all of said plurality of burners to a group of burners which contains at least half but fewer than all of said burners, in which group all burners are at a lower elevation than all burners not in said group.
 12. A method according to claim 8 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.
 13. A method according to claim 8 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners.
 14. A method of modifying a steam generation apparatus, comprising (A) providing a steam generation apparatus comprising (1) a combustion chamber into which gaseous oxidant and fuel are fed, and in which said fuel and said gaseous oxidant are combusted thereby producing heat and hot flue gas, (2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber, (3) a pathway into which water is fed, from which heated steam is obtained at an outlet, and within which the water is evaporated and converted to said heated steam by heat transferred thereto in one or more heat exchangers in said pathway and within said steam generation apparatus to which heat of said combustion is radiated and in one or more convective heat exchangers in said pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred by convective heat exchange, the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam, and (4) a second pathway into which steam is fed, from which said steam is obtained at an outlet, and within which said steam is heated by heat transferred thereto in one or more additional heat exchangers in said second pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred, the second pathway further comprising one or more devices that can pass coolant into steam in said second pathway to reduce the temperature of said steam therein, wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and (B) increasing the rate at which steam is provided at said outlet at a temperature within said given temperature range to a mass flow rate higher than said highest mass flow rate, by (1) increasing the mass flow rate into said pathway of water by an amount equal to the desired increased flow rate of steam and increasing the mass flow rate of fuel into said combustion chamber by an amount corresponding to the increased amount of heat needed to produce said steam at said increased rate, while (2) increasing the oxygen content of said gaseous oxidant with which said fuel is combusted but decreasing the volumetric flow rate at which said gaseous oxidant having increased oxygen content with which said fuel is combusted is fed into said combustion chamber to values effective to provide said increased mass flow rate of steam without exceeding any of said maxima, and (3) combusting said fuel in said combustion chamber with said gaseous oxidant having said increased oxygen content fed at said decreased volumetric flow rate, without decreasing or increasing the heat transfer area of any of said heat exchangers, wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.
 15. A method according to claim 14 wherein said gaseous oxidant fed into said combustion chamber in step (A)(1) is air.
 16. A method according to claim 14 wherein said fuel is coal.
 17. A method according to claim 14 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.
 18. A method according to claim 14 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising feeding more than half of the total amount of fuel that is fed to all of said plurality of burners to a group of burners which contains at least half but fewer than all of said burners, in which group all burners are at a lower elevation than all burners not in said group.
 19. A method according to claim 14 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.
 20. A method according to claim 14 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners.
 21. A method of operating a steam generation apparatus, comprising (A) providing a steam generation apparatus comprising (1) a combustion chamber into which gaseous oxidant and fuel are fed, and in which said fuel and said gaseous oxidant are combusted thereby producing heat and hot flue gas, (2) a preheater into which and through which said hot flue gas passes for transferring heat to said gaseous oxidant before said gaseous oxidant is fed to said combustion chamber, (3) a pathway into which water is fed, from which heated steam is obtained at an outlet, and within which the water is evaporated and converted to said heated steam by heat transferred thereto in one or more heat exchangers in said pathway and within said steam generation apparatus to which heat of said combustion is radiated and in one or more convective heat exchangers in said pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred by convective heat exchange, the pathway further comprising one or more devices that can pass coolant into steam in the pathway to reduce the temperature of said steam, and (4) a second pathway into which steam is fed, from which said steam is obtained at an outlet, and within which said steam is heated by heat transferred thereto in one or more additional heat exchangers in said second pathway and within said steam generation apparatus to which heat in said hot flue gas is transferred, the second pathway further comprising one or more devices that can pass coolant into steam in said second pathway to reduce the temperature of said steam therein, wherein the highest mass flow rate at which said apparatus can provide steam at said outlet at a temperature within a given temperature range, when said gaseous oxidant is air and said air is the only source of oxygen for said combustion, is limited by the maximum flue gas temperature to which said heat exchangers can be exposed, by maximum rates at which said fuel and gaseous oxidant can be fed into said combustion chamber and at which said flue gas can be withdrawn therefrom, and by the maximum rate at which said coolant can be passed into said steam, and (B) providing steam at said outlet at a temperature within said given temperature range at a desired mass flow rate which is higher than said highest mass flow rate, by (1) feeding water into said pathway at a mass flow rate equal to the desired mass flow rate of steam and feeding fuel into said combustion chamber at a mass flow rate corresponding to the amount of heat needed to produce said steam at said desired mass flow rate, while (2) feeding into said combustion chamber gaseous oxidant with which said fuel is combusted having a higher oxygen content than air, at a volumetric flow rate lower than the volumetric flow rate of air at which said highest mass flow rate of steam is provided when air is the only source of oxygen for said combustion, at values of said oxygen content and said volumetric flow rate which are effective to provide said desired mass flow rate of steam without exceeding any of said maxima, and (3) combusting said fuel in said combustion chamber with said air and said gaseous oxidant having said increased oxygen content fed at said lower volumetric flow rate, wherein the heat transfer area of said heat exchangers is the same as when air is the only source of oxygen for said combustion, and wherein flue gas produced in said combustion chamber that exits from said apparatus is not fed back into said apparatus, and the temperature of said flue gas entering said preheater is lower than when said flue gas is produced by combustion in said apparatus when air is the only source of oxygen for combustion.
 22. A method according to claim 21 wherein said fuel is coal.
 23. A method according to claim 21 wherein steam is provided at said outlet at a temperature within said given temperature range at said increased oxygen content and decreased volumetric flow rate of said oxidant without passing any coolant into said steam.
 24. A method according to claim 21 wherein said gaseous oxidant and fuel are combusted in said combustion chamber at a plurality of burners, and at least one of said burners is at a higher elevation than at least one other burner, the method further comprising feeding more than half of the total amount of fuel that is fed to all of said plurality of burners to a group of burners which contains at least half but fewer than all of said burners, in which group all burners are at a lower elevation than all burners not in said group.
 25. A method according to claim 21 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, and all oxidant necessary for complete combustion of said fuel is fed through said burners.
 26. A method according to claim 21 wherein said gaseous oxidant and fuel are combusted in said combustion chamber in one or more burners, wherein a portion of the oxidant necessary for complete combustion of said fuel is fed through said burners and the balance of the oxidant necessary for complete combustion of said fuel is fed into said combustion chamber through one or more overfire ports separate from and vertically above said one or more burners, provided that the oxygen content of the oxidant fed through said one or more overfire ports is less than the oxygen content of the oxidant fed through said one or more burners. 